**1. Introduction**

Wine fermentation is a complicated biochemical process in which yeasts play an active role in the production of ethanol, CO2, and other metabolites from glucose and fructose of grapes [1]. Wine fermentation spontaneously takes place by yeast strains that are present on the grape surface or winery equipment. By today's technology, to achieve complete fermentation, good oenological properties and high production yield commercially produced yeast strains, and mostly *Saccharomyces cerevisiae* are used for wine fermentation as starter microorganisms [2].

*S. cerevisiae* strains derived from industrial wine have hexose transporters (HXT 1–7) that are responsible for wine fermentation. It is mentioned that there is no growth or fermentation when HXT 1–7 are deleted from the genes of this yeast [3]. Ethanol formation in the wine medium causes a change in the affinities of hexose transporters and the change in the affinities of hexose transporters causes stuck fermentation. In this study, stuck fermentation due to ethanol formation was discussed and the experiments were conducted to prevent stuck fermentation.

The hexose sugars, glucose and fructose, are the main reducing monosaccharides present in grapes or grape musts. The amounts of total sugars in grapes or grape musts change between 160 and 300 g/L that consist of almost equal amounts of glucose and fructose before fermentation [4]. During wine fermentation, yeasts, especially *S. cerevisiae*, coferment these monosaccharides and produce wine components [3–5]. Since yeasts have glucophilic character, which is the preference of fermenting glucose to fructose [5], the utilization rate of glucose is higher than that of fructose during fermentation [4]. The glucophilic character of yeasts may be due to transportation across the plasma membrane of yeast by hexose transporters or phosphorylation inside the cell of yeast by hexose kinases has different affinities through glucose and fructose [4]. These different utilization rates result in glucose/ fructose discrepancy (GFD) and residual fructose amount higher than 2 g/L [6] when the fermentation process is completed [4]. Since the sweetness of fructose is approximately twice than that of glucose [7], it affects the final sweetness of wine and the wine fermentation results in higher sweetness, which is undesirable in the wine industry. Also, high residual fructose increases the risk of microbial spoilage [8–10] and decreases the ethanol yield in wine [4–6]. This has been informed that sluggish (incomplete) or stuck (depleted) fermentation in the literature [6].

Although the exact reason for stuck or sluggish fermentations has not been determined yet, there are more than 15 reported reasons such as nitrogen deficiency [3, 4, 6, 8, 9, 11], limitation or excess amount of oxygen [8, 9], too much clarification [8, 9], formation of by-products due to fermentation [9], high ethanol accumulation [4, 6, 8, 9, 12], vitamin and mineral deficiency [8, 9], toxic residues for yeasts from fermentation [5, 8, 9], deprivation of nutrients for yeasts [10], too high or low temperatures [10], environment with high acidity [10], the formation of inhibitors like phenols [10], change in the equation of ionic components [10], higher sulfite content [8], and so on.

There are some known reasons for stuck fermentation and also, there are some possible ways and improvement methods against stuck fermentation such as nitrogen supplementation, controlling the oxygen amount, controlling the temperature of the environment, selecting the yeast according to process, controlling nutrients for yeast growth, and so on. Although many techniques and improvements are developed, stuck fermentation is still a major problem for the wine industry since it causes product losses [9].

Usage of an enzyme in wine media for preventing or restarting the stuck fermentation was not studied before. Therefore, using glucose isomerase to prevent or restart the stuck fermentation was studied as a novel approach.
